netdev/ipv6: Move xxx_ipv6multicast from arch to common code

The `xxx_ipv6multicast` function in each driver is not adapted to
multiple IPv6 addresses yet, and they're redundant, so try to take them
into common code.

Change:
1. Add MAC `g_ipv6_ethallnodes` and `g_ipv6_ethallrouters` in
   `icmpv6_devinit` and call them in `netdev_register`
2. Add multicast MAC for Neighbor Solicitation when adding any IPv6
   address, and remove them when IPv6 address is removed
3. Select `NET_MCASTGROUP` when `NET_ICMPv6` because now we need
   `d_addmac` when we have ICMPv6

Note:
We want modules outside net stack to call functions like
`netdev_ipv6_add` and never touch the related MAC address, so these MAC
functions are added as internal functions to `net/netdev/netdev.h`

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

wireless/ieee802154/mac802154_netdev.c

@@ -843,6 +843,7 @@ static int macnet_addmac(FAR struct net_driver_s *dev,
    *  Not used with IEEE 802.15.4 radios.
    */
 
+  UNUSED(priv);
   return -ENOSYS;
 }
 #endif
@@ -874,6 +875,7 @@ static int macnet_rmmac(FAR struct net_driver_s *dev, FAR const uint8_t *mac)
    *  Not used with IEEE 802.15.4 radios.
    */
 
+  UNUSED(priv);
   return -ENOSYS;
 }
 #endif